Process for the Preparation of Alcohols from Alfa, Beta-Unsaturated Aldehydes and Ketones

ABSTRACT

A chemoselective process for producing alcohols from α,β-unsaturated aldehydes and ketones is described.

The present invention relates to fragrance ingredients and to methods of forming same. In particular the present invention relates to a method for producing alcohols from 60 ,β-unsaturated aldehydes and ketones.

The reduction of 60 ,β-unsaturated aldehydes is an important reaction, because the possible products, such as saturated aldehydes, and saturated or unsaturated alcohols, have found many uses in fragrances, cosmetics, and pharmaceutical industries; also they constitute relevant intermediates in the synthesis of various fine chemicals. Due to the presence of two or more moieties with similar reactivity the reaction is complicated, in particular, when it is desired that not all moieties react.

Methods of producing alcohols from 60 ,β-unsaturated aldehydes are reported in literature. For example, P. J. Baricelli et al. (Catalysis Letters 2011, 141, 709-716) describe a biphasic hydrogenation of 60 ,β-unsaturated aldehydes with a hydrosoluble ionic ruthenium pre-catalyst. Depending on the conditions, mixtures of aldehydes, saturated alcohols and unsaturated alcohols are obtained. Alternatively, alcohols from 60 ,β-unsaturated aldehydes can be produced by transfer hydrogenation in mixed solvents as described, for example, by Zhao et al. (Applied Organometallic Chemistry 2010, 24, 763-766). Unfortunately the known processes lack the selectivity with regard to the similar reactivity of the carbonyl group and C═C bonds.

However, for a process to be useful in an industrial context, in particular in fragrance industry, it must achieve the required high chemoselectivity and also proceed with full, or substantially full, conversion of the substrate.

Thus there remains a need for an industrially acceptable process for the preparation of alcohols from 60 ,β-unsaturated aldehydes and ketones further containing one or more di- or trisubstituted olefin carbon-carbon double bond(s), which proceeds with high conversion and with high chemoselectivity for the double bond in 60 ,β-positon, without hydrogenation of remaining C═C bond(s).

Surprisingly, applicants found that a two-step process comprising heating 60 ,β-unsaturated aldehydes/ketones containing at least one additional di- and/or trisubstituted carbon-carbon double bond, in the presence of a homogenous catalyst, a base and hydrogen permits a conversion of the 60 ,β-unsaturated aldehydes/ketones selectively into their respective saturated alcohols, without hydrogenation of the remaining double bond(s).

Thus there is provided in a first aspect a process comprising the steps of:

(a) heating to about 30-70° C. (preferably about 40-60° C. (e.g. about 45° C., about 50° C. or about 55° C.) a compound of formula (II)

 in the presence of hydrogen gas, a base and a homogenous ruthenium catalyst resulting a compound of formula (Ill)

(b) followed by increasing the temperature up to about 100-150° C. (preferably about 110-130° C. (e.g. about 115° C., about 120° C. or about 125° C.), resulting in a compound of formula (I)

wherein R¹ is hydrogen or methyl;

R² is hydrogen or methyl;

R³ is methyl, or ethyl;

R⁴ is hydrogen or methyl; and

R is a linear or branched C₃-C₁₅ alkyl comprising at last one (e.g., 2, or 3 double bonds) non-terminal carbon-to-carbon double bond (such as 3-methyl but-2-enyl, 3,7-dimethylocta-2,6-dieneyl, 2,3-dimethylbut-2-enyl, but-2-enyl, 2,4,7-trimethylocta-2,6-dieneyl, 2-methylpent-2-enyl)

By the term “non-terminal carbon-to-carbon double bond” is meant either a di-substituted vicinal carbon-to-carbon double bond, a tri-substituted carbon-to-carbon double bond, or a tetra-substituted carbon-to-carbon double bond.

Non-limiting examples are compounds of formula (II) wherein R¹ is hydrogen.

Further, non-limiting examples are compounds of formula (II) wherein R¹, and R² are hydrogen.

Further, non-limiting examples are compounds of formula (II) wherein R¹ is hydrogen and R² is methyl.

Further, non-limiting examples are compounds of formula (II) wherein R¹ is hydrogen and R²=R³.

Further, non-limiting examples are compounds of formula (II) wherein the α,β-unsaturated double bond is preferably in E- configuration.

Further, non-limiting examples are compounds of formula (II) wherein R³ and R⁴ are methyl, and the α,β-unsaturated double bond is preferably in E- configuration.

Further, non-limiting examples are compounds of formula (II) wherein R is a branched C₅-C₁₅ alkyl (including C₆, C₇, C₈, C₉, C₁₀ and C₁₁ alkyl) comprising at least one (e.g., 2, or 3 double bonds) tri-substituted carbon-to-carbon double bond.

Further, non-limiting examples of compounds of formula (II) are compounds selected from 3,7-dimethylocta-2,6-dienal (R is C₅-alkyl comprising one tri-substituted double bond); 2,4,7,11-tetramethyldodeca-2,6,10-trienal (R is C₁₀-alkyl comprising two tri-substituted double bonds); 2,3,6,7-tetramethylocta-2,6-dienal (R is C₆-alkyl comprising one tetra-substituted double bond); 2,4-dimethylocta-2,6-dienal (R is C₄-alkyl comprising one vicinal di-substituted double bond); 3,7,11-trimethyldodeca-2,6,10-trienal (R is C₁₀-alkyl comprising two tri-substituted double bonds); 3,6,8,11-tetramethyldodeca-2,6,10-trienal (R is C₁₁-alkyl comprising two tri-substituted double bonds); and 2,4,6-trimethylnona-2,6-dienal (R is C₆-alkyl comprising one tri-substituted double bond).

The ruthenium catalyst employed in the present invention may be any of those disclosed in EP1970360, which document, for the purpose of disclosing the ruthenium catalysts contained therein, is hereby incorporated by reference.

In a particular embodiment of the present invention, the process proceeds in the presence of a homogeneous ruthenium (II) catalyst. The term “homogeneous catalyst” is well known to the person skilled in the art, and refers to catalysts which are present in the same phase as the substrate.

In one embodiment the homogeneous ruthenium (II) catalyst is a complex (A)

with two triarylphosphines, a diamine and two electronegative ligands (e.g. X may be a halogen such as Cl, Br, and I).

In one specific embodiment the ruthenium (II) catalyst is a compound of formula (A) wherein Ar is para-methyl phenyl.

In another specific embodiment the ruthenium (II) catalyst is RuCl₂(PPh₃)₂(ethylene-diamine), which is commercially available or may be prepared in-situ from commercially available reagents by a process as described, for example, by Takeshi Ohkuma et al. (J. Am. Chem. Soc. 1995, 117, 10417).

To be of high commercial value, the molar ratio between substrate (compound of formula (II)) and the catalyst is at least 500:1, preferably between about 1 000:1 and about 10 000:1, or even higher.

To obtain high yields, step (b) follows after substantially all substrate (II) has been converted to compounds of formula (III), for example, 88% or more (including 90, 92, 95, 98, 99%) of the substrate has been converted, as measured by gas chromatography (GC) relative peak area. For example, by 50% conversion is meant that the peak area of the starting material (compound of formula (II)) is reduced by 50% under the same GC conditions. The conversion can be measured by techniques known in the art. For example, it can be measured by taking a sample of the reaction mixture after a given time and analyzing said mixture by GC analysis. Optionally, one may also monitor the reaction by nuclear magnetic resonance (NMR) spectroscopy or thin layer chromatography (TLC).

In addition to the high chemoselectivity of the processes described herein, a further advantage of the process is that it can be performed almost without solvent. The only solvent necessary is to assist in the solubility of the base. Suitable solvents include alkyl alcohols, such as methanol, ethanol, iso-propanol, n-butanol.

Suitable bases include alkali metal compounds selected from the group consisting of hydroxides (such as sodium hydroxide, lithium hydroxide, potassium hydroxide, cesium hydroxide and the like), alcoholates (such as sodium methoxide, potassium ethoxide, and the like), carboxylates (such as sodium acetate, potassium acetate and the like), carbonates and hydrogencarbonates including sodium carbonate, sodium hydrogen carbonate, lithium carbonate, potassium carbonate, cesium carbonate and the like.

In one embodiment the process of the invention is carried out in the presence of a base selected from sodium hydroxide, sodium methoxide and potassium hydroxide.

The invention is now further described with reference to the following non-limiting examples. These examples are for the purpose of illustration only and it is understood that variations and modifications can be made by one skilled in the art.

EXAMPLE 1

Under air atmosphere NaOMe (30wt % in MeOH) (19.8 g, 4.0 wt %, 0.11 mol) was added to a 1 L stainless steel autoclave containing RuCl₂(PPh₃)₂(ethylenediamine) catalyst (0.197 g, 0.26 mmol), 2,4,7-trimethylocta-2,6-dienal (94.2% purity, E/Z, 50:1) (499.2 g, 3.00 mol) and i-PrOH (10 g). While stirring with an overhead stirrer the autoclave was flushed three times with N₂, then three times with H₂, then put under H₂ pressure (41.0 bar) and heated to 50° C. After 3.9 hours almost full C═O conversion was observed (>95.5%) by GC (relative peak area) resulting in 2,4,7-trimethylocta-2,6-dienol (compound of formula (III)). The temperature of the autoclave was raised to 120° C. (temperature reached within 0.2 h). After a further 3.9 h the reaction was stopped, cooled to room temperature, the pressure released and the autoclave was flushed three times with N₂. The yellow homogeneous mixture was washed with water (210 mL). The aqueous phase was extracted with methyl tertbutyl ether (350 mL). The organic layer was washed with brine (100 mL), water (100 mL) and dried over MgSO₄ (50.0 g), filtered and concentrated in vacuo. To the obtained yellow liquid paraffin oil (12.3 g) was added and after distillation using a Vigreux column, a colorless liquid of 2,4,7-trimethyloct-6-en-1-ol was obtained (426.2 g, 92.4% purity, 81.9 mol % chemical yield, boiling point=75° C. at 0.5 mbar). In addition to the main product, traces of the intermediate 2,4,7-trimethylocta-2,6-dienol, and double bond isomers thereof have been identified at low concentrations, such as (E)/(Z)-2,4,7-trimethyloct-3,6-dien-1-ol, and (E)/(Z)-2,4,7-trimethylocta-4,6-dienal.

EXAMPLE 2

A 10 mL glass vial was charged with 1 g of 2,4,6-trimethylnona-2,6-dienal (99% purity, E/Z, 32:1), 1 mL cyclohexane, 2.1 mg of Cl₂Ru(PPh₃)₂(ethylenediamine) and 210 μL of NaOMe (30 wt % in MeOH). The vial was equipped with a stirring bar and sealed. The vial was flushed with N₂ (1 bar) and then three times with H₂ (1 bar) before setting the pressure of H₂ (40 bar). The reaction mixture was heated at 50° C. for 2 h and then 120° C. for a further 2 h. Once cooled, the H₂ overpressure was purged and the vial flushed with N₂. The reaction mixture was quenched with H₂O, the layers separated and the aqueous phase extracted twice with MTBE. The combined organic phases were washed with brine, dried over MgSO₄, filtered and solvent removal provided crude alcohol which was purified by distillation using Kugelrohr (T=100° C., t=15 min, p=0.2 mbar) to provide a colorless liquid of 2,4,6-trimethylnon-6-en-1-ol (450 mg, 82% purity, 36.9 mol % chemical yield).

COMPARISON EXAMPLE 1

Under air atmosphere NaOMe (30wt % in MeOH) (19.8 g, 4.0 wt %, 0.11 mol) is added to a 1 L stainless steel autoclave containing RuCl₂(PPh₃)₂(ethylenediamine) catalyst (0.197 g, 0.26 mmol), 2,4,7-trimethylocta-2,6,dienal (94.2% purity, E/Z, 50:1) (499.35 g, 2.78 mol) and i-PrOH (10 g). While stirring with an overhead stirrer the autoclave is flushed three times with N₂, then three times with H₂, then put under H₂ pressure (41.0 bar) and heated to 120° C. After 22 hours almost full C═O conversion is observed (>95.5%) measured by GC (relative peak area). The reaction is stopped, cooled to room temperature, the pressure released and the autoclave is flushed three times with N₂. The yellow homogeneous mixture is washed with water (210 mL). The aqueous phase is extracted with methyl tertbutyl ether (350 mL). The organic layer is washed with brine (100 mL), water (100 mL) and dried over MgSO4 (50.0 g), filtered and concentrated in vacuo. To the obtained yellow liquid is added paraffin oil (12.3 g) and after distillation using a Vigreux column, a colorless liquid of 2,4,7-trimethyloct-6-en-1-ol is obtained (298.5 g, 89.1% purity, 69 mol % chemical yield). 

1. A process comprising the steps of: (a) heating to about 30-70° C. a compound of formula (II)

 in the presence of hydrogen gas, a base and a homogeneous ruthenium catalyst and thereby forming a compound of formula (III)

(b) followed by increasing the temperature up to about 100-150° C., and thereby forming a compound of formula (I)

wherein in each of formula (I), (II) and (III): R¹ is hydrogen or methyl; R² is hydrogen or methyl; R³ is methyl, or ethyl; R⁴ is hydrogen or methyl; and R is a linear or branched C₃-C₁₀ alkyl comprising at last one non-terminal carbon-to-carbon double bond.
 2. The process according to claim 1 wherein the base is an alkali metal compound.
 3. The process according to claim 1 wherein the temperature is increased after at least 88% of the compound of formula (II) has been converted to the respective alcohol of formula (III).
 4. The process according to claim 1, wherein the catalyst is a homogeneous ruthenium (II) catalyst.
 5. The process according to claim 4 wherein the homogeneous ruthenium (II) catalyst is a Ru complex comprising two triarylphosphines, a diamine and two electronegative ligands.
 6. The process according to claim 1, wherein the ruthenium catalyst is RuCl₂(PPh₃)₂(ethylene-diamine) or RuCl₂(P(para-methyl phenyl)₃)₂(ethylene-diamine).
 7. A process according to claim 1 wherein the α,β-unsaturated double bond of the compound of formula (II) is in E- configuration.
 8. A process according to claim 1 wherein: R¹ is hydrogen; R² is hydrogen; R³ is methyl; R⁴ is methyl; and R is selected from the group consisting of 3-methyl but-2-enyl, 3,7-dimethylocta-2,6-dieneyl, 2,3-dimethylbut-2-enyl, but-2-enyl, 2,4,7-trimethylocta-2,6-dieneyl, and 2-methylpent-2-enyl.
 10. A process according to claim 8 wherein R is 3-methyl but-2-enyl. 